Method of cathodically sputtering a layer of silicon having a reduced resistivity



, Aug. 26, 19 69 M. BLo oM 3,463,715

METHOD OF CATHODICALLY SPUTTERING A LAYER OF SILICON HAVING A'REDUCEDRESISTIVITY Filed July '7, 1966 I 14 I I7 I N VEN TOR.

I Mare/my 84.00/14 V AUDI-"AMY METHOD OF CATHODICALLY SPUTTERING A LAYEROF SILICON HAVING A REDUCED RESISTIVITY Murray Bloom, Los Angeles,Calif., assignor to TRW Inc., Redoudo Beach, Calif., a corporation ofOhio Filed July 7, 1966, Ser. No. 563,482 Int. Cl. C23c 15/00 US. Cl.204-192 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a method ofsputter depositing semiconductor materials such as silicon on asubstrate to produce a silicon layer having high conductivity in boththe longitudinal direction parallel to the surface of the substrate andin a direction normal or perpendicular to the substrate surface. Themethod comprises the steps of sputter depositing the silicon in areduced pressure atmosphere to form a silicon layer on the substrate andthereafter heating the deposited layer in an environment of purehydrogen preferably at a temperature of about 1,000 C. for approximately15 minutes.

This invention relates generally to methods of fabricating semiconductordevices and more particularly to improvements in methods employingsputtering for deposit ing semiconductor material such as silicon.

Various sputtering devices suitable for depositing semiconductormaterial, such as silicon, are known in the art. Essentially all ofthese devices are comprised of a cathode, formed of the material to bedeposited, and an anode supported within an enclosed sputtering chamber.A substrate upon which the sputtered material is to be deposited is alsosupported within the chamber. The chamber is initially evacuated and agas, such as argon, is then let in. The gas is then ionized by asufficiently high potential applied between the cathode and anode. At anappropriate gas pressure, the potential will maintain a discharge withinthe chamber. The discharge is due to a multiplication process in whichthe collision between positive ions and gas atoms produces more positiveions and electrons. The positive ions are, of cOurse attracted to thecathode and the electrons to the anode. En route, they each collide withgas atoms thus producing more positive ions and electrons. When thepositive ions finally reach the cathode, they strike with such energythat atoms of the cathode material are ejected and collect on thesubstrate. By properly controlling various parameters such astemperature, pressure, duration, etc., several characteristics of thedeposition canbe controlled. a

It has been found that although sputtering can be employed to yieldepitaxial silicon layers suitable for certain applications, certaincharacteristics of the deposited layers make them unsuitable for otherapplications. More particularly, silicon layers obtained as a result ofsputtering usually exhibit poor longitudinal (i.e. parallel to thesurface) conductivity but very good conductivity in a direction normalto the surface. Consequently, although sputtering has been used in orderto fabricate diodes which do not require good longitudinal conductivity,sputtering has heretofore not been particularly useful for formingtransistor bases for example, which require conductivity both normal tothe surface and longitudinally.

In view of the foregoing, it is an object of the present invention toprovide a method of forming silicon layers by sputtering, which layersexhibit good conductivity both normal to the surface and longitudinally.

Briefly, in accordance with a preferred method of practicing theinvention, epitaxial silicon layers exhibiting United States Patent 03,463,715 Patented Aug. 26, 1969 good conductivity both normal andparallel to their surfaces are achieved by heating the deposited siliconlayer in an oxygen-free environment after deposition by sputtermg.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionwill best be understood from the following description when read inconnection with the accompanying drawings, in which:

FIGURE 1 is a sectional view illustrating a typical sputteringapparatus;

FIGURE 2 is an enlarged cross-sectional view illustrating two layers ofsilicon wherein the upper layer appears to be comprised of isolatedcrystallites; and

FIGURE 3 is a sectional view illustrating a substrate having silicondeposited thereon being heated in accordance with a preferred method ofthe present invention.

Attention is now called to FIGURE 1 which illustrates a sectional viewof a sputtering apparatus 10 suitable for depositing semiconductormaterial such as silicon, on a substrate 12 supported (by means notshown) within the apparatus 10. Typically, the substrate 12 comprises anassembly of semiconductor devices and as an example, the top surfacethereof can be formed of silicon. It should of course be appreciatedhowever that the substrate could comprise other structures such as aslice of semiconductor material or a single crystal of sapphire, forexample.

More particularly, the sputtering apparatus 10 is comprised of acylindrical housing or chamber 14 formed of a conductive material, forexample aluminum. An anode 16 is supported within and electricallyconnected to the housing by a conductive stem 17. A cathode 18 is alsodisposed in the housing spaced from and opposed to the anode 16. Thecathode is formed of a material of the conductivity type and resistivitydesired to be deposited on the substrate. A conductive stem 22physically and electrically connected to the cathode 18, extends throughan opening 24 in the end wall of the housing 14. The stem 22 isinsulated from the housing 24 by a vacuum-tight insulating collar 26. Aportion 28 of the housing projects inwardly around the cathode 18 toshield the cathode and stem 22.

*In addition to the foregoing, the housing 14 is provided with an inletaperture 30 and an exhaust aperture 32. The exhaust aperture 32 is usedto evacuate the housing. The inlet aperture 30 is used to enable gas tobe leaked into the housing.

As an example, consider that it is desired to deposit silicon of aparticular conductivity type and resistivity onto the substrate 12. Inorder to do this, a cathode 18 having the desired conductivity type andresistivity is selected. A gas such as argon is permited to enter thechamber through aperture 30 with the pressure within the chamberremaining substantially constant. An appropriate pressure can forexample be approximately forty microns of mercury. A potential of 2000volts or more is applied between the anode and cathode. This potentialsets up a discharge between the anode and cathode which is due to amultiplication process in which collisions occur between positive ionsand gas atoms which collisions produce more positive ions and electrons.The positive ions are drawn to the cathode and strike it with sufiicientenergy to eject therefrom atoms of the cathode material which collect onthe substrate 12 forming a layer of the cathode material.

As a quantitative example, silicon can be sputtered from a .01 ohmcentimeter N type silicon cathode onto a polished P type silicon waferin an atmosphere of pure argon by utilizing a 5000 volt anode-cathodepotential. In one hour, at a pressure of fifty microns, a silicon layerhaving a thickness of .8 micron can be deposited. Although such a layerwill exhibit good conductivity normal to the surface (i.e. from the topto the bottom surface of the layer), typically, layers deposited bysputtering as herein described, exhibit virtually no longitudinalconductivity (i.e. parallel to the layer surfaces). This fact can beconfirmed by contacting the deposited layer with first and second probesspaced longitudinally from one another. A battery source and an ammetercan be connected in series between the probes. Usually, there will be nosignificant current flow between the probes demonstrating a very poorcurrent conducting characteristic in the longitudinal direction. Thischaracteristic is probably attributable to the fact that the depositedlayer is formed of isolated crystallites.

More particularly, FIGURE 2 illustrates an enlarged View of a typicalsubstrate 12 having a layer of silicon 34, deposited by sputtering,affixed thereon. It is believed that the deposited silicon layer 34 iscomprised of oriented crystallites 36 which are separated by walls ofsilicon dioxide 38 which arise as a result of oxygen impurities.

In accordance with the present invention, the substrate 12 and depositedlayer 34 of FIGURE 2 are heated in an oxygen-free environment in orderto improve the longitudinal conductivity characteristic of the layer 34.For example, the substrate '12 can be supported on a graphite susceptor40 supported within an induction furnace 42 heated by coil 44.Preferably, the furnace should be filled with hydrogen inasmuch ashydrogen will combine with any residual oxygen to assure an oxygen-freeenvironment. The temperature of the deposited layer 34 should then beelevated to a level below the melting point of silicon and thistemperature should be maintained for a duration dependent upon thetemperature level. That is, the use of a higher temperature of a shorterinterval will have substantially the same etfect as the use of a lowertemperature for a longer interval.

It has been found that after the deposited layer 34 has been heated to1000 C., for example, for a period of about fifteen minutes, itdemonstrates a markedly increased conductivity in the longitudinaldirection. For example, a deposit which displayed no detectableconductivity prior to heating displayed a sheet resistance of only 2000ohms per square after being heated. As a consequence, utilization of asputtering deposition technique together with a subsequent heating stepas demonstrated by FIGURE 3 enables transistor bases, for example, whichrequire good longitudinal conductivity to be formed by sputtering. It isbelieved that the conductivity improves as a consequence of theelimination of the silicon dioxide walls 38 by a chemical reactionbetween the silicon dioxide and either excess silicon or hydrogen.

From the foregoing, it should be appreciated that a method has beendisclosed herein for depositing epitaxial silicon layers by sputteringwhich layers exhibit good current conductivity characteristics in bothlongitudinal and transverse directions thus enabling them to be used tofabricate transistor bases, for example, where these characteristics arerequired.

What is claimed is:

l. A method of depositing a layer of silicon on a substrate, said methodincluding the steps of:

sputtering atoms from a cathode formed of the silicon desired to bedeposited by accelerating said atoms toward said substrate by anelectric field;

collecting said sputtered atoms on said substrate to thus form a layerof said silicon thereon; and

heating said layer in an oxygen-free environment which is inert withrespect to silicon at a temperature in the range of about 1000 C. to atemperature below the melting point of silicon but above 1000 C. for atime sufficient to reduce the resistivity to a fixed minimum value atthe temperature within said range.

2. The method of claim 1 wherein said layer is heated to temperatures ofapproximately 1000 C. in a pure hydrogen environment.

3. The method of claim 1 including the additional step of supportingsaid cathode in an enclosed chamber and wherein said step of sputteringincludes the step of establishing a discharge in said chamber.

4. A method of increasing the conductivity of a sputter depositedsilicon layer in a direction parallel to the surfaces thereof includingthe steps of:

placing said layer in an oxygen-free environment which is inert withrespect to silicon; and

heating said layer to temperatures of approximately 1000 C. for a timeof approximately 15 minutes.

5. The method of claim 4 wherein said layer is heated in an environmentof pure hydrogen.

References Cited UNITED STATES PATENTS 3,021,271 2/1962 Wehner 204-1923,323,954 6/1967 Goorissen 204164 3,325,392 6/1967 Rummel 204l92 ROBERTK. MlHALEK, Primary Examiner US. Cl. X.R.

